Fuel injection devices for operating an internal combustion engine have been generally known for many years. In what is termed a common-rail injection system the fuel is delivered into the respective combustion chamber of the internal combustion engine by means of injectors, in particular by means of piezoelectric injectors. In this case a high injection pressure is advantageous, since by this means it is possible to achieve a high specific performance of the internal combustion engine on the one hand and low pollutant emissions on the other. When a high-pressure pump and a pressure accumulator are used for the fuel, injection pressures of 1600 to 1800 bar can be attained.
Metering of the fuel quantity by means of a piezoelectric injector is effected by varying the opening and closing times of its nozzle needle. It is well known that the electrical and mechanical characteristic variables which control said nozzle needle are subject to a considerable fluctuation in terms of temperature and/or aging. A precise control of the nozzle needle is advantageous because by that means a more accurate fuel injection is made possible. Furthermore, the very act of applying energy to the piezoelectric injector also leads to an influencing of the electrical characteristic variables. In order, therefore, to maintain the quality of the combustion at all times it is necessary to compensate for the above-cited effects on the fuel injection by means of a control loop.
A method for performing a functional diagnosis of a piezoelectric injector and regulating the fuel injection into an internal combustion engine is known from DE 103 36 639 A1. In this case the piezoelectric injector can be controlled in a charge- or voltage-based manner. It is charged up by applying a specified electrical voltage, and the amount of charge resulting at said voltage is compared with a reference charge amount that is to be expected at said voltage. The functional capability of the piezoelectric injector and/or the power output stage is deduced from the difference between the charge amounts.